The invention relates generally to combustion in burners of principally hydrocarbon-air fuel mixtures, and in particular to methods and apparatus for increasing the efficiency and flame stability of such burners.
The burners typically have fuel transported through a pipe that terminates in a nozzle in the burner combustion chamber. Air is supplied to the area of the nozzle. Combustion is typically initiated by electrodes in the same area.
While such burners, under ideal conditions, can be made to operate efficiently, several factors occur to reduce over-all efficiency. First, burners have limited turn-down ratio (ratio of maximum to minimum heat energy output) resulting in poor match between the burner and the load to be heated. This requires frequent burner shut-off which reduces the over-all efficiency, due to the loss of heat through the large heat capacity of the intermediary heating equipment.
Second, some fuels, principally low BTU fuels, have low flame temperature and are therefore more susceptible to quenching (they are more unstable).
Third, conventional methods of preheating fuel while in the pipe are known to increase the initial rate of vaporization of fuel droplets in the spray leaving the nozzle and to therefore improve combustion. However, there are limits to the amount of pre-heating which can be accomplished while the fuel is confined in the pipe. Chemical decomposition tends to occur for heated fuel, causing part of the fuel to solidify and become attached to pipe and nozzle surfaces. When this happens, the heat transfer rate through the pipe decreases, creating a need for still higher pipe temperature and further compounding the problem.
Additionally it is known that preheating the fuel before injecting it to the burner produces vaporization of the molecules with the highest energy first, which in turn results in the droplet temperature dropping rapidly to below the temperature of the surrounding gas. When the droplet temperature becomes low enough to accept enough heat by radiation from the flame front and by conduction from the surrounding gas, its temperature stabilizes and its rate of evaporation is controlled by these heat transfer mechanisms in a conventional burner. This phenomenon is especially important when the burner output is low. This is true because at low output levels the droplet and all velocities are low and part of a droplet can pass out of the small flame cone and not be vaporized by the flame, resulting in part of the fuel not being combusted. This leads also to the formation of deposits on burner surfaces, increased emissions, and reduced burner heat transfer efficiency. Having less fuel consumed for a given air flow results in lower flame temperatures and attendant lower burner efficiency.
Fourthly, it is known that burners operate more efficiently when running slightly lean (with slightly excess air) because of both more complete combustion and lower emission of smoke. Under rich combustion, emitted smoke coats the heat exchanger surfaces, reducing heat transfer and allowing more heat to escape up the chimney as well as requiring more frequent maintenance. However, since burner flames are more unstable under lean operation and more susceptible to be blown out by variation in environmental conditions, they are typically operated at slightly rich air-fuel ratios (with excess fuel), therefore wasting fuel and producing smoke.
It is clear that it would be desirable to preheat fuel and vapor entering a burner combustion chamber besides electrically stimulating the resulting combustion for optimum operation. Prior art references teach the application of microwave energy to the flame front of a combustion region of an internal combustion engine to stimulate burning. They include (1) Ward, U.S. Pat. No. 3,934,566, where it is shown that for internal combustion the flame-front electron plasma frequency and the electron-neutral collision frequency are in the microwave frequency range and thus have the correct properties insofar as allowing microwave energy to be coupled to the flame-front plasma; (2) Ward application Ser. No. 622,165, where it is shown that use can be made of the metal combustion chamber to improve coupling to the flame front by exciting combustion chamber resonant cavity modes; (3) Ward U.S. Pat. No. 4,138,980 where it is shown that for a typical combustion chamber of the conventional internal combustion engine type, microwave power levels of the order of 100 watts are sufficient to significantly heat flame-front electrons and improve combustion, and (4) Ward application Ser. No. 968,376, where it is shown that for optimal stimulation of the flame, the combustion chamber must be reshaped and/or the microwave mode chosen so that the highest electric fields are maintained at the region of the initial flame front or the region where the flame is most likely to become unstable.
It is an object of the invention to apply microwave energy to burners in ways that will make their combustion more efficient.
It is also an object of the invention to produce a better way to preheat fuel and improve the vaporization for such burners to produce better flame stability and more complete combustion, even at low burner power outputs.
It is another object of the invention to stimulate the combustion region in such burners to increase their flame stability, allowing combustion to occur in a smaller volume and at leaner mixtures.
It is another object of the invention to heat fuel in the pipe leading to such burners solely and indirectly by the application of microwave energy in ways that can complement microwave stimulation of the combustion region. It is another object of the invention to provide for heating of the fuel vapor, or spray, resulting from heated fuel itself.
Other objects of the invention are to increase the efficiency and reliability of combustion in a hydrocarbon fuel burner by methods and apparatus that involve little additional expense and uncomplicated operation.
Other objects and advantages of the invention will be pointed out hereinafter or be readily apparent from the following discussion.